The majority of urinary tract stones are formed of calcium. However, there are rarer forms of stone disease which include cystinuria, primary hyperoxaluria, Adenine phosphoribosyltransferase deficiency (APRT) and Dent’s disease. As with other rare conditions there is often a delay before the underlying diagnosis is made. These stones types are important to diagnose as they often lead to recurrent stones, pain and multiple operations. If left untreated they can cause infections and damage to the kidney leading to hypertension and renal impairment. In addition to the surgical treatment of the stone, there are also medications available for prevention once the diagnosis has been made.
Cystinuria has the highest incidence of these rare stones. It is caused by a defect in an amino acid transport mechanism in the kidney. This leads to an accumulation of cystine, ornithine, lysine and arginine in the urine. Cystine is the only clinically important one as it is not soluble in these higher levels whereas the others are. The treatment of cystinuria is fluids, diet, alkalinisation and thiol binding drugs all of which aim to make the cystine more soluble and reduce stone formation.
Primary hyperoxaluria is caused by a defective enzyme in the oxalate metabolic pathway which leads to accumulation of oxalate. This often leads to renal failure and kidney transplantation.
APRT causes radiolucent stones and kidney failure if untreated, however, the treatment is simple with allopurinol or febuxostat.
Dents disease is due to a defect in the proximal tubule of the kidney leading to hypercalciuria. Rare inherited stone disorders should always be considered in the differential diagnosis of urinary stones that present in childhood or adolescence, and adults with unclear causes of recurrent stone disease. A high index of suspicion, along with early diagnosis and appropriate treatment, can help prevent kidney injury and perhaps the serious long-term complications of rare stone disorders.